This application claims the benefit of a Japanese Patent Application No.2002-333501 filed Nov. 18, 2002, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference. This application is also based on a Japanese Patent Application No.2002-173620 filed Jun. 14, 2002, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to optical amplifiers and optical amplifier control methods, and more particularly to an optical amplifier for amplifying optical signals (light signals or signal lights) and an optical amplifier control method for controlling such an optical amplifier.
2. Description of the Related Art
Recently, techniques for manufacturing quartz optical fibers having a low loss on the order of 0.2 dB/km, for example, and techniques for utilizing such optical fibers, have been established. Hence, optical communication systems utilizing such optical fibers as transmission lines have been reduced to practice. In such an optical communication system, an optical amplifier is provided to amplify the optical signal, so as to compensate for the loss of the optical fiber and enable a long-distance transmission.
A conventional optical amplifier includes an optical amplifying medium which receives the optical signal to be amplified, and a pumping unit which pumps (or excites) the optical amplifying medium so as to provide a gain band including a wavelength of the optical signal.
For example, an Erbium (Er) Doped Fiber
Amplifier (hereinafter simply referred to as an EDFA) has been developed as one example of an optical amplifier for amplifying an optical signal which has a wavelength of the 1.55 μm band and small loss in the quartz optical fiber.
The EDFA includes an Erbium (Er) Doped
Fiber (hereinafter simply referred to as an EDF) as the optical amplifying medium, and a pump light source for supplying pump light having a predetermined wavelength to the EDF. The EDFA uses pump light having a wavelength in the 0.98 μm band or the 1.48 μm band, so as to obtain a gain band including a wavelength of 1.55 μm.
A Wavelength Division Multiplexing (WDM) is a technique for increasing a transmission capacity of the optical fiber. In the optical communication system which is applied with the WDM, a plurality of optical carriers having different wavelengths are used, as proposed in a Japanese Laid-Open Patent Application No.11-122192, for example.
In the optical communication system applied with the WDM, a plurality of optical signals obtained by independently modulating each of the optical carriers are wavelength-division-multiplexed by an optical multiplexer, and a resulting WDM optical signal is supplied to an optical fiber transmission line. At a receiving end, the received WDM optical signal is demultiplexed into individual optical signals by an optical demultiplexer, and transmission data are reproduced based on each of the optical signals. Accordingly, in the optical communication system applied with the WDM, the transmission capacity of one optical fiber is increased depending on the number of optical signals which are multiplexed.
In other words, the optical amplifier is used as a linear repeater in the optical communication system applied with the WDM. For this reason, compared to a case where a conventional reproducing repeater is used, it is possible to reduce the number of parts within the repeater and secure reliability of the repeater, and also reduce the cost of the repeater.
When assembling the optical amplifier in the optical communication system applied with the WDM, various controls need to be made with respect to the optical amplifier, due to the necessity to maintain a wavelength characteristic of the gain constant and to prevent waveform deterioration due to non-linear effects of the optical fiber transmission line.
For example, in the EDFA, the wavelength characteristic of the gain changes depending on the gain which is determined by the pumping condition, and thus, an Automatic Gain Control (AGC) is carried out so as to produce an output having a predetermined gain with respect to the input. In this case, if the input changes under the predetermined gain, the output accordingly changes.
On the other hand, from the point of view of a signal-to-noise (S/N) ratio, it is desirable for the optical amplifier to produce a high signal output. However, if the waveform deterioration due to the non-linear effects of the optical fiber transmission line and an input dynamic range at the receiving end are taken into consideration, it is not always desirable for the optical amplifier to produce a high signal output. In other words, there are demands to carry out an Automatic Level Control (ALC), so that the output of the optical amplifier becomes constant within a predetermined range.
As a suitable structure for realizing both the AGC and ALC, an optical amplifier has been proposed which includes first and second optical amplifier units and a variable optical attenuator connected between the first and second optical amplifier units. According to this proposed optical amplifier, the AGC is carried out in each of the first and second optical amplifier units, and the ALC is carried out by the variable optical attenuator.
Such an optical amplifier has been proposed for the following reasons. First, from the point of view of optimizing a Noise Figure (NF) of the entire optical amplifier, it is disadvantageous to provide the variable optical attenuator for the ALC at a preceding stage. Second, from the point of view of securing a predetermined signal output power of the optical amplifier, if the variable optical attenuator for the ALC is provided at a subsequent stage, it is necessary to obtain a high signal output power in the optical amplifier unit for the AGC at an immediately preceding stage, but this is disadvantageous from the point of view of realizing a lower power consumption of a laser diode which is used as the pump light source.
In the optical amplifier having the structure which is suited for realizing both the AGC and the ALC as described above, there is a problem in that the structure of the optical amplifier becomes complex because of the need to independently carry out the AGC in each of the first and second optical amplifier units.
In addition, when using the optical amplifier in the optical communication system applied with the WDM, there is a problem in that the control of the variable optical attenuator for the ALC is complex if a number of channels of the WDM changes. More particularly, when carrying out the ALC to amplify the WDM optical signal in the optical amplifier, a control is carried out so that the total power of the output of the variable optical attenuator becomes constant. Hence, if the number of channels of the WDM optical signal changes during operation of the optical communication system, a target value of the control of the variable optical attenuator becomes different.
The target value of the control of the variable optical attenuator is generally supplied from a monitoring control unit which is provided on an upstream side, and a complex monitoring operation becomes necessary if the wavelength of the optical communication system changes. Moreover, although the attenuation of the variable optical attenuator is temporarily fixed when the wavelength of the optical amplifier changes, it is necessary to carry out operations such as updating the target value of the control depending on the change in the wavelength in a state where an ALC loop is released and closing the ALC loop again, thereby introducing a possibility that the attenuation quantity (amount of attenuation) of the variable optical attenuator will vary during the series of operations.
Since the AGC is carried out continuously in the first and second optical amplifier units, there is a possibility that the output power will vary per wavelength channel when the target value of the control of the variable optical attenuator is switched.